The reduction in aerodynamic drag resulting from the addition of a boattail shaped appendage to a boxed shaped vehicle has been well examined. Drag reductions of 30% are realistically possible (NASA/TP-1999-206574, “A Reassessment of Heavy-Duty Truck Aerodynamic Design Features and Priorities”), and would correspond to fuel savings of 15% for a transport truck. There are however several practical requirements that pose difficulties to implementing such a device on a working transport trailer. The device must not impede the operation of the trailer doors, typically swinging double doors on trailers used for long-distance haulage. The device must present a smooth, gently tapering set of boattail (side) and backlight (top) surfaces that connect tightly to the trailer sides and top respectively for optimum aerodynamic performance. Given practical difficulties in tightly fitting an inflatable to the hardware laden trailer door perimeter, the present invention uses rigid shelves to bridge the gap that exists between the trailer's trailing edge and the inflated bags when the bags are mounted onto the doors away from the edges and door hinges. This arrangement is also advantageous in eliminating undesirable surface contours due to the inflatable bag's surface bulge that occurs in the transition zone where the airfoil meets the trailer. Not only must the device not impede door operation, but it must get out of the way of the doors without any significant dismantling effort on the part of the operator. Given that the device operates in a physically hostile environment, it must not vibrate to a degree that causes premature wear of its structural components. It should automatically collapse when the trailer stops so that it is not battered by high winds which could attack it directly from the side while stopped, unlike its' relatively wind sheltered position while travelling at highway speed.
The present design comprises dual ram-air inflated bags as a framing structure for the overlying airfoil surfaces. The force of the inflated bag internal pressure against the trailer doors is counterbalanced by the hingedly attached hardware that attaches the airfoil to the trailer, thereby drawing the hardware tight and minimizing its' vibration during airfoil operation. The inflation/deflation cycle occurs slowly and calmly over several minutes. Both of these aspects relating to the inflated bags minimize wear on the attachment hardware which is necessary for such a device to last several years without need for major structural maintenance. An inflatable however, requires surface shape control and stiffening in order to remain stable during operation, and avoid surface vibration and wear. To this end, the present disclosure describes a series of staggered interrupted ribs attached along the inflated bags surfaces that provides surface stability during inflation, while accommodating complex folding for stowage.
Transport trailers operate in conditions of cross wind creating an angle of yaw at which the air attacks the trailer, typically in the 5–10 degree range. Boattail surfaces tend to perform aerodynamically better in cross-wind (higher yaw) conditions, however the backlight loses its' drag reducing function as the angle of yaw rises. In order to preserve backlight performance in yaw, the present device provides for a vertical fin to redirect trailer roof air flow directly behind the trailer where it can contribute to pressure recovery as it would in the zero yaw condition. The present device overcomes practical operating difficulties of a fin such as structural integrity(flexibility) during high cross-wind conditions while still allowing for automatic deployment and stowage.
Given that the transport trailer is a road vehicle, drag relating to underbody airflow as well as flow around the rear wheels near the ground figures significantly in overall drag. The present device overcomes difficulties in practically cleaning up the airflow in this domain in order to approach theoretical minimum drag. To this end the present disclosure further describes a set of panels that provide for ground domain streamlining of airflow. A major difficulty in the deployment of rigid or semi-rigid panels as airfoil surfaces behind a trailer is the weight and complexity of adequate structural reinforcement, particularly if the panels are large enough to perform well aerodynamically and are not collapsed automatically when the trailer stops, thereby exposing them to severe ambient wind gusts. The present device overcomes these problems by anchoring the panels to the inflated bags, and by using bag surface tension to lift and lock panel support members into position and dampen their vibration. Graham (U.S. Pat. No. 6,854,788 B1) has used radially oriented collapsible tension bearing struts to stabilize the boattail surfaces. These struts are disposed and function in a manner similar to the radially oriented cords inside the inflatable bags of Andrus (U.S. Pat. No. 6,409,252 B1), the difference being that Graham creates boattail surface tension (that offsets strut tension) by bending semi-rigid flexible sheets for boattail surfaces, whereas Andrus uses air pressure within the inflatable bags. The panel support members of the present disclosure differ from the tension bearing radially oriented cords/struts of Andrus (U.S. Pat. No. 6,409,252 B1) and Graham (U.S. Pat. No. 6,854,788 B1) in that they are rigid folding struts that lock into position when the boattail panels are deployed thus fixing the panels against movement both laterally and medially, and therefore do not depend upon boattail surface tension to be stiffened. The main advantage of this approach over that of Graham (U.S. Pat. No. 6,854,788) is that the panels can be easily collapsed automatically when the airfoil is not in use without having to open the trailer doors. The panels as disclosed herein therefore do not depend on trailer door closure in order for them to assume an aerodynamically effective angle relative to the side of the trailer. Examples of scenarios in which automatic collapse of the panels would be preferable include stopping or city driving when severe ambient winds prevail in order to avoid wind damage, parking in a trailer yard in order to avoid parking damage and space usage, and having a damaged airfoil during long distance delivery such that it is not safe to deploy yet not near repair shop.